Induction type crucible furnace

ABSTRACT

A crucible furnace of the induction type in which the melting pot is supported at the inner side of an induction coil which in turn is surrounded by circumferentially distributed transformerlamination packets which are forced against the coil and hence also against the pot wall by radially adjustable clamping devices connected by screw-threaded joints to correspondingly circumferentially located upright posts forming components of the mounting base of the furnace. To prevent undesirable flow of stray currents induced in the support structure for the melting pot including the lamination packets, the clamping devices and their threaded connections to the upright posts, which are usually made of iron, electrical insulation is inserted between each clamping device and the associated lamination packet loaded by it.

United States Patent Hegew aldt et al.

[ INDUCTION TYPE CRUCIBLE FURNACE [75] Inventors: Fritz Hegewaldt, Herdecke; Hans Werner Mauve, Lunen-Brambauer; Otto Erdmann, Dortmund, all of Germany [73] Assignee: BBC Brown Boveri & Company Limited, Baden, Switzerland [22] Filed: Mar. 5, 1974 [21] Appl. No.: 448,407

[30] Foreign Application Priority Data May 13, 1975 Primary ExaminerR. N. Envall, Jr. Attorney, Agent, or Firm-Pierce, Scheffler & Parker [57] ABSTRACT A crucible furnace of the induction type in which the melting pot is supported at the inner side of an induction coil which in turn is surrounded by circumferentially distributed transformer-lamination packets which are forced against the coil and hence also against the pot wall by radially adjustable clamping devices connected by screw-threaded joints to correspondingly circumferentially located upright posts forming components of the mounting base of the furnace. To prevent undesirable flow of stray currents induced in the support structure for the melting pot including the lamination packets, the clamping devices and their threaded connections to the upright posts, which are usually made of iron, electrical insulation is inserted between each clamping device and the associated lamination packet loaded by it.

4 Claims, 1 Drawing Figure PATENTED HAY I 31975 INDUCTION TYPE CRUCIBLE FURNACE The present invention relates to an improvement in the construction of a crucible furnace of the induction type wherein a ceramic melting pot is supported at the inner side of a cylindrical induction coil and wherein transformer-lamination packets distributed around and in contact with the outer side of the coil in uniformly spaced relation are forced against the coil by radially adjustable clamping means which are connected by screw-threaded joints to upright posts forming components of the mounting base of the furnace.

Such a construction is utilized for large furnaces of the forementioned type and wherein a tilting frame is utilized in conjunction with the mounting base and the furnace components supported by it and in such manner that the mounting base and hence the melting pot can be tilted by conventional hydraulic or pneumatic means so as to facilitate discharge of the molten mass in the pot from its pouring lip.

The induction coil is electrically insulated on all sides and its winding turns are secured in place in the axial direction by means of end and intermediately located spacer members. The exterior surface of the melting pot is characterized by asbestos layers which engage the inner side of the coil. These asbestos layers are applied during manufacture of the melting pot at the time the wet ceramic mass from which it is molded is being compressed. During the drying phase, the moisture within the ceramic mass is driven outward through these asbestos layers. Afterwards, these asbestos layers function as thermal insulation and as a temperature control for the ceramic to maintain the gradual sinterization with the desired limits.

An insulating interlining is placed between the outer side of the current-carrying coil and the transformerlamination packets in order to galvanically separate the coil from the lamination packets which are at ground potential and which surround the coil in circumferentially spaced relation.

As explained hereinabove, clamping means are utilized in conjunction with each lamination-packet to apply a radically inward pressure to the assembly of the melting pot and the coil surrounding it thereby to hold it securely in place. Each such clamping means is comprised of a press head which is forced radially inward against the lamination packet by means including a threaded connection with the corresponding upright post of the mounting base. Since these upright posts are rigid, the screwthreaded interconnection enables the effective length of the clamping means terminating in the press head to be adjusted thereby effecting a corresponding adjustment in the pressure applied by it to the melting pot. Moreover, the clamping means are resilient within themselves in order to ensure a continuous contact pressure between the coil and the melting pot which latter will vary in dimension with variations in the temperature level and will therefore lead to the development of cracks in the wall of the pot due to the outward pressure thereon from the molten mass within it if it is not supported at all times against the inner surface of the coil.

The transformer-lamination packets also serve to guide the magnetic field created by current flow through the coil and which extends into the air outside the melting pot. Since the magnetic return path of the transformer laminations is not complete, one portion of the magnetic field will induce a voltage in those parts of the mounting base for the furnace that are made from iron. Since the furnace components form closed circuits by way of the clamping means with the transformer-lamination packets which are arranged superposed as well as spaced uniformly side-by-side at the periphery of the coil, currents will flow within all of these components which correspond to the stray field and the induced voltage. Such current will cause arcing at the points of current transfer, i.e. at the threaded joints incorporated in the clamping means, thus causing the latter to bind and preventing one from adjusting the clamping force from time to time as the occasion arises and also preventing one from measuring the magnitude of the clamping force by suitable tools such as torque keys. Inability to perform such adjustments will lead to damage to the furnace during operation since in the case where the contact pressures from the lamination packets become insufficient, the insulation on the coil will become damaged due to excessive widening of the coil, and cracks will develope within the ceramic melting pot due to insufficient support.

The principal objective of the present invention is to improve the prior art construction as noted in the foregoing example by preventing flow of stray currents induced in the support structure for the melting pot and, in particular, flow of any such currents through the threaded interconnections embodied in the adjustable clamping means thereby eliminating any possibility of creating a binding condition and thus assuring easy and unobstructed movement of these interconnections at all times during operation of the furnace for proper adjustment and re-adjustment of the contact pressure exerted by the clamping components against the transformer-lamination packets.

The invention solves this problem in that electrical insulation is placed between each clamping means and the associated lamination-packet loaded by it. For reasons of simplicity of manufacture, the electrical insulation is provided by use of an oxide ceramic. In a case where a particularly strong current flow between a transformer-lamination packet and the clamping device associated therewith must be prevented, it is proposed that for reasons of operational safety dual layers of insulation on component parts which contact each other be utilized, i.e. an insulation layer on each such part. In the event that tilting forces which cannot be precisely determined exist between a transformerlamination packet and its associated clamping device and are required to be balanced, it is proposed that both a molded plastic and an oxide ceramic in contact with each other be used for the desired electrical insulation.

It is also possible, in order to simplify manufacture, to select various methods of providing the electrical insulation, depending upon the repair of an existing furnace or construction of a new one, to spray an oxide ceramic coating onto the thrust bearing and/or pressureexerting member of the clamping means, or to make one or both of these components from an oxide ceramic. It is further possible, in accordance with a further development of the invention to manufacture a plate component located intermediate the outer surface of the lamination packet and thrust bearing from a molded plastic or an oxide ceramic.

The foregoing as well as other objects and advantages inherent in the improved clamping structure for the melting pot and the induction coil surrounding it will become more apparent from the following detailed description of a suitable embodiment and the accompanying drawing, the single FIGURE of which is principally a sectional view at one side of the melting pot showing a portion of the wall thereof and coil together with one of the circumferentially distributed transformerlamination packets and the clamping device associated therewith as well as the upright post of the mounting base which carries that particular clamping device.

With reference now to the drawing, a portion of the wall 2 of the melting pot 1 made from a compressed ceramic mass is seen to include at the periphery thereof a cylindrical asbestos interlining 4 against which the induction coil 3, insulated on all sides, is placed. Coil 3 is made from a hollow conductor material, and a fluid coolant is circulated through it by means of a cooling system as is conventional. The individual turns of the coil are fixed in place in the axial direction by end and intermediate spacing components in the customary manner. However, these details have not been included since they are not essential to an understanding of the present invention.

A cylindrical insulation lining 5 lies in contact with the periphery of coil 3 and the system of transformerlamination packets 6 which are placed uniformly about the circumference of the melting pot at the level of coil 3 lie in contact with the periphery of the lining 5. The lamination packets serve to guide the magnetic flux which flows outside the melting pot and are connected to ground potential.

Secured to the outer surface of the lamination packet 6 is a plate 7 which serves to support a thrust bearing 8 which is provided with a recess 9 for receiving a press head 10 which is secured to the radially inner end of a cylindrical support 10a which makes a telescoped fit within a cylindrical portion 12a at one end of a clamping member 12 and which is threaded at 12b for engagement with a threaded opening 13a provided in the corresponding upright post 13 of a grounded mounting base for the melting pot and induction coil assembly which has not been detailed. These upright posts are distributed in uniform spaced relation around the circumference of the pot and coil assembly and opposite the lamination packets 6.

Numeral 18 designates a cover member fastened to the thrust bearing 8 and which surrounds and captures a portion of the bulbous press head 10 so as to prevent the latter from dropping out of recess 9 prior to installation and adjustment of the clamping device.

Surrounding the cylindrical support 10a for the press head 10 adjacent an end flange thereon are a pair of cup springs 14 which are engaged by the end of the cylindrical portion 12a of the clamping member 12 so that as the latter is rotated by means of the screw threaded connection joint 12b, 1311, it will advance longitudinally in the direction of the cup springs 14, thus compressing these to provide a resilient pressure connection with the flanged end of the cylindrical support 10a and press head 10 thus transmitting a corresponding pressure to the thrust bearing 8 which in turn transmits pressure through support plate 7, lamination packet 6, interlining 5 and coil 3 to wall 2 of the melting pot 1. Each such clamping device, designated overall by numeral 11 and extending radially from the melting pot 1 is thus seen to be adjustable longitudinally in the direction of pot l by rotation of the threaded part 12 within the upright post 13, thus to vary the pressure exerted inwardly against the pot wall and coil in such manner as may be dictated to be necessary during operation of the furnace to provide a proper support for these components.

As has already been stated, since the magnetic return path of the transformer laminations is not complete, one portion of the magnetic field produced by coil 3 will induce a voltage in those components of the I mounting base which are made of iron and, since some of those components form closed circuit paths by way of each clamping device with the transformerlamination packets, induced current will flow through each of the clamping devices 11 and will lead to arcing at any point of current transfer from one part to another, e.g. the threaded connections 12b, 13a causing these to bind.

To prevent this from taking place, the present invention proposes that electrical insulating material be inserted between each clamping device 1 l and the corresponding transformer-lamination packet against which it applies its pressure. One way in which this insulation can be interposed in the path over which induced current would otherwise flow is to cover the press head 10 with a layer 15 of an oxide ceramicwhich can be applied by conventional spray technique, and to similarly coat the surface of recess 9 of the thrust bearing 8 with an oxide ceramic 16. In lieu of an oxide ceramic, the recess 9 can be lined with a molded plastic 17. These alternatives have been indicated by correspondingly different hatching at different parts of the recess.

As another alternative, the desired electrical insulation can be established by making the entire thrust bearing and press head from an oxide ceramic. These alternatives have been designated on the drawing by the use of numerals 8 and 10'.

Support plate 7 can be made of metal or an oxide ceramic or a molded plastic.

The invention is particularly advantageous in that the galvanic separation of the transformer-lamination packets from their combined supporting and clamping devices, i.e. by interposing electrical insulation, will prevent flow of stray currents between the threaded components of the clamping devices and the upright support posts of the mounting base for the furnace, thereby assuring that proper adjustments and readjustments of the pressures established by the clamping device relative to the transformer-lamination packets can be maintained throughout the life of the furnace and which, because of this improvement, is substantially lengthened.

We claim:

1. In a crucible furnace of the induction type the combination comprising a ceramic melting pot, said melting pot being supported at the inner side of an induction coil, transformer-lamination packets distributed around and in contact with the outer side of said coil in uniformly spaced relation, support plates secured along the periphery of said transformerlamination packets, a thrust bearing secured at the side of each said support plate, each said thrust bearingbeing constructed as an electrical insulator and provided with a recess in which a press head engages and including a cover member which surrounds and captures a portion of said press head, and each said press head being constructed as an electrical insulator and disposed at the end of a radially adjustable clamping lating oxide ceramic layer and said thrust bearings are provided with an electrically insulating molded plastic layer at their mutual engagement surfaces.

4. A crucible furnace as defined in claim 1 wherein said support plates for the transformer-lamination packet are made from an electrically insulating material. 

1. In a crucible furnace of the induction type the combination comprising a ceramic melting pot, said melting pot being supported at the inner side of an induction coil, transformerlamination packets distributed around and in contact with the outer side of said coil in uniformly spaced relation, support plates secured along the periphery of said transformer-lamination packets, a thrust bearing secured at the side of each said support plate, each said thrust bearing being constructed as an electrical insulator and provided with a recess in which a press head engages and including a cover member which surrounds and captures a portion of said press head, and each said press head being constructed as an electrical insulator and disposed at the end of a radially adjustable clamping means threadedly engaged with an upright post forming a component of the mounting base of the furnace.
 2. A crucible furnace as defined in claim 1 wherein said press heads and said thrust bearings are provided with electrically insulating oxide ceramic layers at their mutually engagement surfaces.
 3. A crucible furnace as defined in claim 1 wherein said press heads are provided with an electrically insulating oxide ceramic layer and said thrust bearings are provided with an electrically insulating molded plastic layer at their mutual engagement surfaces.
 4. A crucible furnace as defined in claim 1 wherein said support plates for the transformer-lamination packet are made from an electrically insulating material. 